Chard goes by a number of names, the most common of which is Swiss Chard.
Chard is harvested at any stage of its life. When young the stems are tender and can be eaten raw in a salad or by themselves. When mature the stems are sometimes tougher. At this point Chard stalks are often cooked. Chard is very perishable.
There are many cultivars of Chard. The stems can be yellow, red or white or something inbetween. Chard has a somewhat bitter taste which will all but disappear when cooked.